How to get to FCurves from specific strip?
I will wait few days before accepting my own answer. Maybe someone else will have a better 
idea.

Due to lack of other answers and request to do so I will give what I am currently using.
I still feel like it is not the best approach, but it as well may be (I don't know Blender's API that well, just started learning).

I've created an operator that takes all selected speed effects and given the original frame rate of clips (I haven't found a way to get that automatically from movie sequences) creates linear interpolation curve for speed effect.
It is is more useful than constant speed value as you can then go and edit curve not having to work out how it should look like to get original speed in your final edit (it also creates meta strip that actually is trimmed correctly so it appears in correct size in VSE, but that part is omitted as it is outside the scope of the question).

def execute(self, context):  
    # filter out non speed effects
    selection = [ seq for seq in context.scene.sequence_editor.sequences if seq.select and seq.type == 'SPEED' ]

    # get render frame rate and calculate how to scale input movie fps
    fc = context.scene.frame_current
    fps = bpy.context.scene.render.fps
    fcurves = bpy.context.scene.animation_data.action.fcurves
    factor = fps / self.original_framerate 

    # remove current fcurves for speed_factor if there are any
    # we create set for faster lookups (at least I'd home it would be faster than naïve approach
    paths_to_remove = set((s.path_from_id("speed_factor") for s in selection))
    for f in fcurves:
        if f.data_path in paths_to_remove:
            fcurves.remove(f)

    for s in selection:
        s.use_default_fade = False
        s.use_as_speed = False
        factor = fps / self.original_framerate 
        final_duration = s.input_1.frame_duration*factor
        final_duration = s.input_1.frame_duration*factor

        # create curve for this speed effect and add two keyframes
        curve = fcurves.new( s.path_from_id("speed_factor") )
        start_key = curve.keyframe_points.insert( s.frame_start, 0 )
        start_key.interpolation = 'LINEAR'
        end_point = curve.keyframe_points.insert( s.frame_start + final_duration, s.input_1.frame_duration )
        end_point.interpolation = 'LINEAR'